1. Field of the Invention
The present invention relates to a liquid crystal driving circuit.
2. Description of the Related Art
In a segment-display type or a simple matrix driving type liquid crystal panel, a common signal and a segment signal are supplied to a common electrode and a segment electrode, respectively, and turning on/off is controlled in accordance with a voltage (potential difference) between both the electrodes, in general.
In these liquid crystal panels, performing time-division driving enables display of more segments (pixels) than the number of output terminals of an IC for driving a liquid crystal. For example, in a liquid crystal panel with the number m of common electrodes and the number n of segment electrodes, performing 1/m duty driving enables display of m×n segments at the maximum. Further, in the time-division driving, 1/S bias driving is performed so that each signal can obtain (S+1) potentials. For example, in FIG. 4 of Japanese Patent Laid-Open Publication No. H10-10491, disclosed is an LCD driving power circuit used for 1/3 bias driving.
Here, a configuration of a common liquid crystal driving circuit that performs time-division driving and an example of an operation thereof are illustrated in FIGS. 7 and 8.
As illustrated in FIG. 7, intermediate potentials V1 and V2 obtained by dividing a power supply voltage V0 (=VDD−VSS) by a resistor R1 to R3 are supplied, in addition to power supply potentials VDD and VSS on a high-potential side and a low-potential side, to a common-signal output circuit 5 and a segment-signal output circuit 7. Therefore, in this liquid crystal driving circuit, 1/3 bias driving (S=3) is performed.
Further, FIG. 8 illustrates an operation of the liquid crystal driving circuit that performs 1/4 duty driving (m=4). As illustrated in FIG. 8, a common signal COMi (1≦i≦m) is at a power supply potential for a 1/4 period and of an intermediate potential for a 3/4 period, in one period T0, and the waveform is shifted by 1/4 period each. On the other hand, a segment signal SEGj (1≦j≦n) is at a potential according to turning on or off of four segments corresponding to segment electrodes to which the signal is supplied.
As such, use of the 1/m duty and 1/S bias driving method enable display of more segments than the number of output terminals of the IC for driving a liquid crystal.
The common electrode to which the common signal COMi is supplied and the segment electrode to which the segment signal SEGj is supplied are capacitively-coupled through liquid crystal, and thus, beard-like spike noise might be generated in one of the signals, which is caused by a change in potential of the other of the signals. Thus, in the liquid crystal driving circuit illustrated in FIG. 7, similarly to FIG. 4 in Japanese Patent Laid-Open Publication No. H10-10491, capacitors C1 and C2 are used as stabilizing capacities so as to absorb the spike noise and to stabilize the intermediate potentials V1 and V2. As illustrated in FIG. 9, such a liquid crystal driving circuit is known that stabilizes the intermediate potentials V1 and V2 using voltage follower circuits configured by operational amplifiers OP1 and OP2, respectively.
However, since the capacitance of the capacitor used as the stabilizing capacity is required to be sufficiently large in accordance with the liquid crystal panel, the capacitor usually results in an external component, which increases a mounting area of a circuit board. On the other hand, since output impedance of the operational amplifier which makes up the voltage follower circuit is required to be sufficiently small, current consumption is increased. Further, if the output impedance is not sufficiently small, as illustrated in FIG. 10, the spike noise Sp is not sufficiently absorbed, which might cause such defective display that an image remains in the liquid crystal panel.
Thus, in order to ensure favorable display quality, the current consumption of the liquid crystal driving circuit and the mounting area of the circuit board are in a trade-off relationship.